In multi-access communication systems, or networks, for interconnecting multiple devices or stations in a communications network, means must be provided for controlling access of the devices to the communications channel. It has previously been proposed to provide a network using a single coaxial cable to interconnect devices for communication with each other. Of course, only one device can transmit at any given time over such a cable, if all devices operate at the same frequency. One form of control which has evolved for such networks is referred to as carrier-sense multiple access with collision detection ("CSMA/CD"). With this technique, each device controls its own access to the coaxial cable channel. Each device which uses the channel interfaces to the cable through a transceiver which includes apparatus for transmitting a signal onto the channel as well as apparatus for receiving a signal from the channel placed thereon by another device's transceiver. The transceivers each include a collision detector for generating a collision signal whenever a signal transmitted on the cable by another transceiver is detected at the same time the transceiver itself is transmitting onto the channel. Each transceiver sends the collision signal back to its host device and in response both hosts stop transmitting. Both then retry transmission after the channel is clear.
Each device that wants to use (i.e., transmit on) the channel, first "listens" to the receiver in its transceiver to hear if any other station is transmitting. If it detects no other host trasmitting, the station starts its transmission, while receiving at the same time. Another station also might start transmitting, though. If that happens, both detect the collision and stop, as noted above. To avoid repeated collisions, each then waits a pseudorandom interval and tries again. On each retry, the pseudorandom delay is increased.
Such a system is illustrated, for example, in U.S. Pat. No. 4,063,220, issued Dec. 13, 1977 to Robert M. Metcalfe et al.
In such CSMA/CD networks, care must be taken to ensure that the failure of one transceiver or one host device does not impair the usability of the network by other stations. Several transceiver failure modes can be foreseen as "bringing down" the network. For example, if a transceiver cannot properly detect collisions, it must not be used to transmit. Similarly, care must be taken to assure that a transmitter or host does not monopolize the channel. Further, since a short circuit in a transmitter or receiver could short out the cable, transceivers must be properly isolated from the coaxial cable.
Accordingly, it is an object of the present invention to provide an improved transceiver for use in such networks, with greatly enhanced reliability and protection against recognizable failure modes which would adversely affect the network.
It is a further object of the invention to provide a transceiver having integral means for simulating collisions to test the collision detection mechanism in its receiver, to insure its proper operation.
Yet another object of the invention is to provide a transceiver whose transmitter and receiver are isolated from the coaxial cable, to protect against transceiver short circuits shorting out the cable.
Still another object of the invention is to provide a transceiver including means for disconnecting the transmitter from the channel in the event the host or transmitter will not stop transmitting.
A further object of the invention is to provide a transceiver using industry standard emitter-coupled logic to facilitate manufacturing.
These and other, further objects, features and advantages of the present invention will be understood from the following description.